Step gages are used for calibrating extremely accurate measuring instruments. A step gage typically comprises a metal block (or blocks) of a known height with opposing, parallel surfaces which are precision lapped to extremely tight tolerances. Since the height of the step gage is known to an extremely high accuracy, a measuring instrument can be calibrated based on the value measured for the height of the step gage.
Known prior art gages comprise a single block of a specified height. The opposing faces of the block are precision lapped to the desired height. In single block gages, the measuring probe can contact opposing faces of the block on the neutral axis of the gage, i.e., the axis parallel to the dimension that is being measured on which the center of gravity of the gage lies. Gages which allow measuring on the neutral axis are advantageous since any bending of the gage assembly due to gravitational forces or other factors has a minimal effect on the height of the gage at its neutral axis. Single block type gages tend to be extremely precise. However, such gages are useful for measuring only a single height, whereas it is commonly desirable to test a measuring instrument at several different heights.
Step gages are known in which a series of blocks are held in non-contacting relation by a gage assembly. The precision lapped surfaces of the blocks comprise the measuring surfaces which the probe contacts. Non-contacting block probes, such as disclosed in U.S. Pat. No. 4,445,276 issued to Voneky et al., not only provide multiple measuring surfaces at several heights along the gage, but also allow the measuring surface to be located on the neutral axis. Such gages, however, are not particularly popular because of the complexity and expense of their design and because they tend to be less accurate than other types of block gages.
Probably the most popular type of step gage is the contacting block or stacked block step gage in which a series of precision lapped blocks are stacked in contacting relationship and held together by either a through-bolt extending through mating holes in the blocks or individual screws which couple each block to a preceding block via mating holes.
U.S. Pat. Nos. 3,162,955 and 3,276,312, both issued to Egli, disclose particular stacked block step gages. U.S. Pat. No. 3,162,955 (hereinafter Egli '955) discloses a block gage assembly comprising multiple blocks, each block being coupled to a preceding block by a "hermaphrodite" bolt. Each block includes a hole which is drilled through the center of the block and countersunk. A bolt slightly longer than the height of the particular block is inserted in the hole so that the threaded distal end of the bolt extends below the bottom surface of the particular block and, when the gage is assembled, extends into the block immediately below it. The holes are countersunk to a depth that will cause the head of the bolt inserted therein to be approximately flush with the top surface of the associated block. The head of each hermaphrodite bolt includes a threaded cylindrical cavity that accepts the distal end of a bolt inserted in the hole in the block placed above it. Each block is assembled to the block below it by screwing the distal end of the hermaphrodite bolt into the cavity in the head of the hermaphrodite bolt which has been inserted in the preceding block.
U.S. Pat. No. 3,276,132 (hereinafter Egli '132) discloses a step gage assembly similar to the one disclosed in Egli '995 except that the bolts inserted into the holes in the blocks do not engage the cavity in the head of the preceding bolt (i.e., they are not hermaphrodite bolts). Instead, the uppermost portion of the hole in each block is internally threaded to engagedly receive an externally threaded insert, which insert is, in turn, internally threaded to accept the threaded distal end of the bolt inserted in the next block in the assembly. Since the bolts need not contact each other, the bolts are relatively shorter than the bolts of Egli '955 and can all be of the same length. According to Egli '132, the advantage of this design is that the compressive force of each bolt is concentrated at the abutting surfaces of the two adjacent blocks, thus reducing the bending moment on the block that would normally be caused by the compressive force of a bolt which extends through the entire block. Egli '132 discloses compressive forces placed on the blocks by the bolts on the order of 100 psi. The Egli '132 patent states that higher compressive forces are undesirable for two reasons. First, high compressive forces cause bending of the gage. Second, the blocks tend to reduce in size due to the elasticity of the material under extremely high compressive forces. In other words, extremely high compressive forces, compress the blocks causing the assembled gage to be shorter than the desired height.
The Egli '955 patent discusses the possibility of lapping the individual blocks of the gage to a size slightly larger than their nominal size so that, when they are subject to the compressive forces of the bolts, the blocks reach their nominal size.
Another example of a step gage is shown in U.S. Pat. No. 2,537,340 issued to Fonda which discloses a gage block comprising three separate block sections, a steel main section and two tungsten carbide end caps which are coupled to the main section by screws which extend through holes in the end caps and engage threaded holes in either end of the main block. The lower portions of the holes in the tip pieces are slightly larger than the screws they accept to allow the tungsten tip pieces to expand at a different rate than the steel main section without warping the tip pieces through thermal stresses within the block.
It is an object of the present invention to provide an improved step gage.
It is a further object of the present invention to provide a step gage with an increased resistance to warping and damage to minimize inaccuracies in the height of the gage.
It is another further object of the present invention to provide a stacked block type step gage wherein the measuring surface is on the neutral axis of the gage.
It is yet another further object of the present invention to provide a step gage with an increased resistance to a bending moment.
It is still another further object of the present invention to provide an extremely accurate stacked block step gage.